Trench drilling apparatuses generally using two drills are well adapted to making trenches that are deep and of relatively narrow width. However, when the apparatus is required to drill into ground that is hard or very hard, such as stone, conventional machines can be found to be inadequate. The drills used at present for overburden have a maximum weight of 60 (metric) tonnes. Since such apparatuses are suspended from the end of hoisting cables, it is their own weight which is transmitted to the drills in order to perform drilling. Unfortunately, in that kind of ground, it can happen that the load required in order to achieve proper drilling lies in the range 100 tonnes to 150 tonnes, or even more. This force depends on the number of tools and the type of tool mounted on the wheels of the drill.
The simplest solution for increasing the weight of the apparatus is to load the frame of the drill. However, that would require a larger capacity hoist to be used and that naturally gives rise to major drawbacks, particularly concerning site organization.
To remedy that drawbacks, proposals have already been made in French patent No. 2 749 333 in the name of the Applicant for a particular type of drilling apparatus that enables trenches to be drilled in hard ground.